The water soluble polyphenols of plants are industrially significant since they occur in fruit juices as well as in extracts, infusions and fermentations of plant products. There is a wide distribution of such polyphenols in the root, stem, leaves, bark and flowers of many plant families.
There are many chemical classes of polyphenols and these include:
(a) Anthocyanins (substituted flavanols) which give the red, blue and purple colour to many flowers, fruit skins and juices as well as foliage. The betacyanin of beetroot is related to the anthocyanins. The anthocyanins are used as food dyes. PA0 (b) Polyhydroxyflavones which are yellow plant dyes. PA0 (c) Isoflavones which are related to the flavones but are less abundant. PA0 (d) Derris insecticides and fish-stunning rotenones which are related to the isoflavones. PA0 (e) Logwood dyes which are related to the flavanols. PA0 (f) Cannabis compounds which are polyphenol derivatives. PA0 (g) Many mold pigments which are polyphenolic xanthones. PA0 (h) Vitamin E which is a monophenolic chromane derivative and is related to the polyphenols. PA0 (i) Thyroxine which is a phenolic acid of importance to animal metabolism, technically it is a derivative of a polyphenol. PA0 (j) Many polyphenolic acids which occur in fruits and barks, particularly in coffee beans and in plant galls. They are regarded is simple analogues of one class of tannins, the ester tannins. PA0 (k) The ester tannins embrace the sugar esters with polyphenolic acids and are used in inks. PA0 (l) The condensed or catechin tannins are related to the anthocyanins and are used for leather tanning. PA0 (m) The bitter principles of hops, humulones, are related to polyphenols. PA0 (n) The humic acids of soils and waters are oxidised polyphenols and form carcinogenic halocarbons on chlorination during chloride disinfecting, affecting public health. PA0 (o) The estrogenic flavanoids of clovers affect sheep breeding. PA0 (i) Polyamides such as nylon and perlon have a large molecular weight distribution. As they are formed as a large solid mass during manufacture it is not possible to remove the low molecular weight fraction before use. When finely chopped and used as an adsorbent, the low molecular weight fraction is somewhat soluble in the liquid being treated. This contravenes food regulations in some countries. However, their greatest deficiency is that they have a very low adsorption efficiency. PA0 (ii) Polyvinylpyrrolidone (PVPP) has high production costs and must be regenerated for reuse. The regeneration involves the use of strong acids and alkalis which may not be completely washed off and which may therefore contravene food regulations. There is a loss of activity when PVPP is regenerated several times. In addition, PVPP swells in water and tends to block the filters. As it can only be made as a fine powder PVPP cannot be used in a column as a multistage process but must be mixed with the liquid being treated and then filtered out as a single stage process. It is not capable of adsorbing high molecular weight polyphenols such as tannins as only the smaller low molecular weight plant phenols can fit inside the fine resin pores. PA0 (iii)Condensation products of polymers of polyamino and/or polyhydroxy compounds and formaldehyde as described in Great Britian Patent 1,531,852 are not specific adsorbents for plant polyphenols. They adsorb acids which are lost from the fruit juice or other drink, casting doubt on the legality of continuing to call the liquid a pure food when some components are missing. Their method of preparation by powdering a large mass results in low surface area and hence low adsorptive efficiency. They cannot be tailored for pore size. PA0 (iv) Polyetherurethanes, as described in European Patent Application 0080298, are nonspecific. They are basically a gummy solvent, removing flavour as well as colour. They have low efficiency.
Polyphenols are present in many commercially important products such as molasses, tea, cocoa, coffee, wine, beer, all fruit juices, herbs and many medicinal infusions.
The anthocyanins are important in wine and fruit juice colours. Their combination with metal salts may discolour these products.
There are many occasions in which fruit juice and wine colours undergo reduction of colour or other modifications by traditional processes such as filtering or settling or adsorbing on non-toxic solids.
The tannins and polyphenolic acids greatly affect the taste, astringency and keeping qualities of wines, beers, fruit juices and especially coffee and tea. The tannins and polyphenolic are also involved in the formation of some hazes and precipitates in beverages.
Traditional methods of removing any of the polyphenolic bodies from one another or even as a class are limited. The higher molecular weight tannins are easiest to remove by precipitation with high molecular proteins such as gelatin, but it is difficult to add the exact amount of gelatin even in favourable cases.
In order to maintain a reasonably constant end product, it is often the case that products from different species and locations are blended together. Although this is a practical solution to product consistency, it is an expensive one.
Food processing has reached the point where there is a need for new procedures for modifying the polyphenol content of a wide variety of products.
For example, the demand for white wines may exceed supply when red wines are in excess. Red wine grapes may be used in part for making white wines. This may be done by excluding the skins but pink tinges often occur and must be removed.
Cheap fruit juice such as apple juice, is often added to aerated waters in place of refined sugars or to other juices to maintain "a wholly natural juice" ingredient claim. The apple browning reaction is rapid in air. Therefore prior to such addition the apple juice must be bleached.
Apple juice is normally bleached with active carbon, but attempts to bleach dark juices can result in reduction of flavour.
Active carbons are used to adsorb brown colours in cane sugar processing and in starch hydrolysates but these brown stains differ from the polyphenol oxidation products in apple juice. A different adsorber from carbon is needed but none has been developed which suits the practical requirements of large scale processing.
In short, market demands have outstripped the traditional food colouring and flavouring practices. The present invention is based upon selective polyphenolic substance removal and recovery which adds a new technique to natural product technology ranging from fine wines to laminated wood.
In conjunction with suitable active carbons, many colours are more completely and more economicaly separated by the processes of the invention to give a colourless liquid with full odour retention and most taste retention although many bitter and astringent tastes can be removed if desired, by removing the causative tannin.
The prior art in polyphenol determination and removal is usually specialised to a particular product such as wine, sugar cane, beer and apple juice. Much of the prior art is ancient in origin and much depends on art and experience, which is often local in application.
However, the main colouring agents of these products are now known and their presence is usually measured at known absorption wavelengths.
Thus the browning of apple juice which is largely a phenolic oxidation, catalysed by a diphenoloxidase, is measured at 420 nanometers by the optical density (O.D.) of a 1 cm cell thickness at 20 degrees Celsius. See D. A. Heatherbell, J. L. Short and P. Strubi, Confructa, Vol. 22 (1977), No. 5/6, p. 158.
The total phenol content is measured by a modified Folin-Ciocalteu procedure following V.L. Singleton and J. A. Rossi Jnr., American Journal of Enology and Viticulture, Vol. 16(1965), p. 144-158.
Recently chromatographic procedures have been developed to analyse these polyphenolic mixtures in somewhat greater detail. However quantitative analysis of fully identified constituents is not possible, because of the presence of polymeric series of homologues and the enormous variations of composition according to conditions of treatment.
Polyphenolic class separations as may be achieved in trace amounts analytically cannot yet be applied on the scale required in food industries.
The analytical adsorption agents used to date have been extremely varied and include a large number of fine powders such as alumina, silica, various clays, bentonites, ion-exchange resins and many types of carbon.
One particular analytical adsorption agent is a polyamide 6,6 powder, which is rather specific for the tannins but not other polyphenols.
Only analytical traces of tannins are adsorbed on chopped filaments of polyamide 6,6 in chromatographic procedures. There is no large scale technology to use such-fine powders as a means of removing tannins from a base material.
Cheap tannin separation would allow tannin from wood wastes to be used in phenolic/formaldehyde wood binding resins. Crude size separation alone by ultrafilters has had some success for glue intermediates but has been inadequate for colour removal and chemical methods are uneconomic.
In summary there has been no versatile large scale process for removing polyphenols from plant extracts, preferably with some separation of chemical classes to allow recovery of valuable polyphenolic components.
Various adsorbents have been proposed but all suffer from difficulties.